traffic-control: Ignore failure to kill wget processes

It’s possible for wget to finish downloading a file before being killed
by the test harness (for example, if the download is particularly fast.

These failures to kill processes don’t indicate failure of the test
case, so should be ignored.

Additionally, prevent client.py from exiting early due to its D-Bus
timeout expiring — tcmmd never sends a reply to the method call, so
client.py should not expect one. This requires ignoring the need for a
reply, and also running the dbus-python main loop correctly rather than
sleeping and potentially ignoring incoming D-Bus traffic.

Bug: https://bugs.apertis.org/show_bug.cgi?id=327
Differential Revision: https://phabricator.apertis.org/D509


Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>